Thallium(III) nitrate mediated ring contraction of 2-aryl-1,2,3,4- tetrahydro-4-quinolones: Stereoselective synthesis of trans methyl 2-aryl-2,3-dihydroindol-3-carboxylates

Article abstract of DOI:10.1080/00397910500466462

The ring contraction of N-acetyl-2-aryl-1,2,3,4-tetrahydro-4-quinolones 1a-d with thallium(III) nitrate in trimethyl orthoformate afforded stereoselectively trans methyl N-acetyl-2-aryl-2,3-dihydroindol-3-carboxylates 5a-d by oxidative rearrangement of ar

Full text of DOI:10.1080/00397910500466462

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Thallium(III) Nitrate
Mediated Ring Contraction of
2‐Aryl‐1,2,3,4‐tetrahydro‐4‐quinolones:
Stereoselective Synthesis of trans Methyl
2‐Aryl‐2,3‐dihydroindol‐3‐carboxylates
Om V. Singh a , M. Muthukrishnan a & M. Sundaravedivelu b
a Phytochemistry Division , Tropical Botanic Garden and Research Institute ,
Pacha‐Palode, Kerala, India
b Department of Chemistry , Ghandhigram Rural Institute , Tamilnadu, India
Published online: 16 Aug 2006.
To cite this article: Om V. Singh , M. Muthukrishnan & M. Sundaravedivelu (2006) Thallium(III) Nitrate
Mediated Ring Contraction of 2‐Aryl‐1,2,3,4‐tetrahydro‐4‐quinolones: Stereoselective Synthesis of trans
Methyl 2‐Aryl‐2,3‐dihydroindol‐3‐carboxylates, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 36:7, 943-950, DOI: 10.1080/00397910500466462
To link to this article: http://dx.doi.org/10.1080/00397910500466462
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Synthetic Communicationsw, 36: 943–950, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500466462
Thallium(III) Nitrate Mediated Ring
Contraction of 2-Aryl-1,2,3,4-tetrahydro-4-
quinolones: Stereoselective Synthesis
of trans Methyl 2-Aryl-2,3-dihydroindol-3-
carboxylates
Om V. Singh and M. Muthukrishnan
Phytochemistry Division, Tropical Botanic Garden and Research
Institute, Pacha-Palode, Kerala, India
M. Sundaravedivelu
Department of Chemistry, Ghandhigram Rural Institute,
Tamilnadu, India
Abstract: The ring contraction of N-acetyl-2-aryl-1,2,3,4-tetrahydro-4-quinolones
1a–d with thallium(III) nitrate in trimethyl orthoformate afforded stereoselectively
trans methyl N-acetyl-2-aryl-2,3-dihydroindol-3-carboxylates 5a–d by oxidative
rearrangement of aryl ring A.
Keywords: Dihydroindoles, oxidative rearrangement, ring contraction, tetrahydro-4-
quinolones, thallium(III) nitrate
Thallium(III) salts are versatile reagents in organic synthesis and have effected
many useful and unique transformations.[1] A comparison of their reactivities
with lead(IV) and mercury(II) toward the oxidation of nitrogen-containing
compounds[2] and also their oxidative rearrangement properties vis a vis
with hypervalent iodine reagents[3] have been reviewed. Recently, we have
demonstrated that the oxidation of 2-aryl-l,2,3,4-tertrahydro-4-quinolones
(1) with thallium(III) salts afforded different products under different reaction
Received in the USA September 16, 2005
Address correspondence to Om V. Singh, Department of Chemistry, University of
Texas at San Antonio, 6900 N. Loop 1604 West, San Antonio, TX 78249, USA.
E-mail: omvir.singh@utsa.edu
943

944
O. V. Singh, M. Muthukrishnan, and M. Sundaravedivelu
conditions[4] as depicted in Figure 1. Thus, oxidation of 1 (R ¼ H) with
thallium(III) p-tosylate (TTS) in dimethoxyethane furnished 2-aryl-4-
quinolone (2), and thallium(III) nitrate (TTN) in trimethyl orthoformate
(TMOF) in presence of HClO4 yielded 2-aryl-4-methoxyquinoline (3).
However, the oxidation of 1 (R ¼ Ac) with TTS or TTN in the presence of
HClO4 in MeCN gave 3-aryl-4-quinolones (4) by rearrangement of the aryl
ring. Herein, we report the oxidation of 1 (R ¼ Ac) with TTN in TMOF in
the presence of perchloric acid.
Treatment of N-acetyl-2-(40-methylphenyl-1,2,3,4-tetrahydro-4-quinolone)
(1b) with TTN in TMOF in presence of HClO4 afforded methyl N-acetyl-2-
(40-methylphenyl)-2,3-dihydroindol-3-carboxylate (5b) as the major product
in 75% yield along with a minor product, 2-(40-methylphenyl)-4-methoxy-
quinoline (3b) as shown in Scheme 1. The compound 5b was formed by the
ring contraction of 1b under reaction conditions whereas compound 3b was
formed by the partial cleavage of N-acetyl group of 1b followed by
oxidation with TTN as reported earlier.[4a] The ring contraction of cyclic
ketones with thallium(III) salts is reported to give acid or ester depending
upon the reaction conditions[5] but not much attention has been paid to the
heterocyclic compounds.
The structure of 5b was confirmed by the analysis of IR, 1H and 13C NMR
and 2D-NMR experiments such as HMQC (heteronuclear multiple-quantum
coherence) and HMBC (heteronuclear multiple bond correlation). The character-
istic features of the 1H NMR spectrum are two doublets of C3 and C2 protons at d
3.93 (J ¼ 1.4 Hz) and 5.78 (J ¼ 1.4 Hz) and the downfield shift of C7 proton at d
8.31 (J ¼ 7.2 Hz) as a result of the deshielding of N-acetyl group. All carbons are
unambiguously assigned with help of HMQC and HMBC experiments. The key
HMBC correlations are as follows: dH 2.05 ! dc 169.49; dH 2.29 !dc 129.89;
dH 3.75 ! dc 171.25; dH 3.93 ! dc 52.80, 138.52, 143.03, 171.25; dH
5.77 ! dc 55.80, 124.97, 169.49; dH 8.31 ! dc 124.10, 126.11.
The stereochemistry of 5b was determined by 1H-1H NOE (nuclear Over-
1
hauser effect) spectrum analysis. The strong H-1H NOEs were observed
Figure 1. Oxidation of tetrahydro-4-quinolones with thallium(III) salts.

Ring Contraction of 2-Aryl-l,2,3,4-Tetrahydro-4-Quinolones
945
Scheme 1.
between C3-H (d 3.93 and ortho protons of C2-aryl ring (d 7.06–7.01, 6%) and
C4-H (d 7.35–7.31, 4%) as shown in Figure 2, which confirms the cis relation-
ship between the C3-H and C2-aryl ring and hence trans stereochemistry of
dihydroindole ring. Furthermore, the trans stereochemistry of the dihydroindole
ring was also confirmed by comparing the coupling constant J2,3 of dihydroin-
dole (5b) with the reported cis and trans 2,3-dihydroindoles. For example, the
coupling constant of cis N-acetyl-2,3-dimethyl-2,3-dihydroindole is 9.5 Hz,
whereas the trans isomer gave only a broad singlet, which may be due to the
low coupling constant of the respective trans isomer.[6]
The generality of this transformation was established by treating different
N-acetyl-2-aryl-1,2,3,4-tetrahydro-4-quinolones (1a–d) with TTN in TMOF
in presence of HClO4 and in all the cases trans methyl N-acetyl-2-aryl-2,3-
dihydroindol-3-carboxylates (5a–d) were obtained as major products in
70–35% yields along with minor amount of 2-aryl-4-methoxyquinolines
(3a–d) in 10–12% yields.
A most plausible mechanistic pathway for this transformation is depicted
in Scheme 2. The ketalization of 1 with TMOF in presence of perchloric acid
furnished ketal 6, which on elimination of methanol gave the intermediate
enol ether 7 as described earlier.[7] The electrophillic attack of thallium(III)
at top face of the double bond of enol ether (anti to C2-aryl ring) would
afford C-thallated adduct 8. The attack of thallium(III) is preferred from the
top face of the double bond because the bottom face is congested because
of the presence of the aryl ring and N-acetyl group. Reductive elimination
Figure 2. 1H-1H NOE’s correlation of 5b.

946
O. V. Singh, M. Muthukrishnan, and M. Sundaravedivelu
Scheme 2.
of thallium(I) from 8 with concomitant migration of aryl ring A from the C4 to
C3 positions would give another intermediate carbocation 9, which on reaction
with water produced the ring-contracted product 5. The migration of the aryl
ring A is preferred over the C2-aryl ring because of the greater stability of
carbocation.
In summary, an efficient and general method has been developed for
the stereoselective synthesis of trans methyl N-acetyl-2-aryl-2,3-dihydroin-
dol-3-carboxylates by TTN-mediated oxidative rearrangement of N-acetyl-
2-aryl-1,2,3,4-tetrahydro-4-quinolones under mild reaction conditions. Trans
methyl N-acetyl-2-aryl-2,3-dihydroindol-3-carboxylates can also be utilized
for the synthesis of azalignans.
EXPERIMENTAL
Melting points were taken in open capillaries and are uncorrected. IR spectra
(nmax in cm21) were recorded on a Perkin Elmer 283 IR spectrophotometer;
1H NMR, 13C NMR, COSY, NOESY, HMQC, and HMBC spectra (chemical
shifts in d ppm) on Brucker DPX-200 (200 MHz) and AMX-400 (400 MHz)
using CDCl3 as solvent; and mass spectra on JEOL JMS-SX 102 mass
spectrometer operating at 70 eV. 2-Aryl-1,2,3,4-tetrahydro-4-quinolones
were synthesized by cyclization of 20-aminochalcones following literature
procedure.[8] TLC was performed on silica gel–coated glass plates and
visualized in UV and/or staining with iodine. All the solvents were distilled
before use.

Ring Contraction of 2-Aryl-l,2,3,4-Tetrahydro-4-Quinolones
947
N-Acetyl-2-aryl-1,2,3,4-tetrahydro-4-quinolones (1a–d), General
Procedure
A solution of 2-aryl-1,2,3,4-tetrahydro-4-quinolones (1 mmol) in acetic
acid–acetic anhydride (1:1; 5 mL) was refluxed for 1 h. The reaction
mixture was cooled to room temperature and concentrated under reduced
pressure to dryness, and the residue was purified by column chromatography
over silica gel using hexane–ethyl acetate to afford N-acetyl-2-aryl-1,2,3,4-
tetrahydro-4-quinolones (1a–d).
N-Acetyl-2-phenyl-l,2,3,4-tetrahydro-4-quinolone (1a): White crystalline
solid, yield 233 mg (88%); mp 164–165 8C (lit.[8c] mp 165–166 8C).
N-Acetyl-2-(40-methylphenyl)-1,2,3,4-tetrahydro-4-quinolone (1b): White
crystalline solid, yield 257 mg (92%); mp 192–193 8C; IR (KBr): v¯ ¼ 1655
(amide), 1685cm21 (.C55O); 1H NMR (200MHz, CDCl3): d 2.29
(s, 3H), 2.40 (s, 3H), 3.19 (dd, 1H, J ¼ 5.5 & 17.9Hz, H-3), 3.33 (dd, 1H,
J ¼ 2.2
& 18.0Hz. H-3), 6.39 (brm, 1H, H-2), 6.97 (brd, 2H,
J ¼ 8.4 Hz), 7.04 (brd, 2H, J ¼ 8.4 Hz), 7.13 (ddd, 1H, J ¼ 7.8, 7.1, &
1.2 Hz) 7.31 (dd, 1H, J ¼ 8.3 & 1.1 Hz), 7.44 (ddd, 1H, J ¼ 8.3, 7.0, &
1.0 Hz), 7.90 (dd, 1H, J ¼ 7.7 & 1.0 Hz); MS (70 eV): m/z ¼ 279 (M+);
anal. calcd. for C18H17NO2: C, 77.40; H, 6.13; N, 5.01; found: C, 77.31; H,
6.19; N, 5.12.
N-Acetyl-2-(40-chlorophenyl)-1,2,3,4-tetrahydro-4-quinolone (1c): White
crystalline solid; yield 275 mg (95%), mp 194–195 8C; IR (KBr): v¯ ¼ 1660
(amide), 1680 cm21 (.C55O); 1H NMR (200 MHz, CDCl3): d 2.42
(s, 3H), 3.19 (dd, 1H, J ¼ 5.5 & 18.0 HZ, H-3), 3.34 (dd, 1H, J ¼ 2.2 &
18.0 Hz, H-3), 6.39 (brm, 1H, H-2), 7.06 (brd, 2H, J ¼ 8.4 Hz), 7.21 (brd,
2H, J ¼ 8.4 Hz), 7.14 (ddd, 1H, J ¼ 7.7, 7.0, & 1.2 Hz), 7.32 (dd, 1H,
J ¼ 8.4 & 1.2 Hz), 7.44 (ddd, 1H, J ¼ 8.4, 7.0, & 1.0 Hz), 7.92 (dd, 1H,
J ¼ 7.8 & 1.0 Hz); MS (70 eV): m/z ¼ 299/301 (M+); anal. calcd.
for C17H14ClNO2: C, 68.12; H, 4.71; N, 4.67; found: C, 68.02; H, 4.82;
N, 4.61.
N-Acetyl-2-(40-methoxyphenyl)-1,2,3,4-tetrahydro-4-quinolone (1d): White
crystalline solid, yield 251 mg (85%), mp 120–121 8C; IR (KBr):
v¯ ¼ 1660 (amide), 1685 cm21 (.C55O); 1H NMR (200 MHz, CDCl3): d
2.39 (s, 3H), 3.16 (dd, 1H, J ¼ 5.4 & 18.0 Hz, H-3), 3.32 (dd, 1H, J ¼ 1.8
& 18.0 Hz, H-3), 3.72 (s, 3H, OCH3). 6.37 (brm, 1H, H-2), 6.69 (brd, 2H,
J ¼ 8.6 Hz), 7.13 (brd, 2H, J ¼ 8.6 Hz), 7.13 (ddd, 1H, J ¼ 7.7, 7.1, &
1.2 Hz), 7.32 (dd, 1H, J ¼ 8.6 & 1.0 Hz), 7.43 (ddd, 1H, J ¼ 8.3, 7.0, &
1.0 Hz), 7.88 (dd, 1H, J ¼ 7.7 & 1.0 Hz); MS (70 eV): m/z ¼ 295 (M+);
anal. calcd. for C18H17NO3: C, 73.20; H, 5.80; N, 4.74; found: C, 73.32; H,
5.93; N, 4.78.

948
O. V. Singh, M. Muthukrishnan, and M. Sundaravedivelu
Oxidation of N-Acetyl-2-aryl-1,2,3,4-tetrahydro-4-quinolones (1a–d)
with Thallium(III) Nitrate in TMOF, General Procedure
A drop of perchloric acid was added to a solution of N-acetyl-2-aryl-1,2,3,4-
tetahydro-4-quinolones (1a–d; 1 mmol) in trimethyl orthoformate (5 mL).
The solution was stirred at room temperature for 15 min. After 15 min
535 mg of thallium(III) nitrate trihydrate (1.2 mmol) was added, and the
reaction mixture was further stirred for 1 h at room temperature. After com-
pletion of reaction, dichloromethane (25 mL) was added, and precipitated
thallium(I) nitrate was filtered off and washed with dichloromethane
(25 mL). Filtrates were combined, transferred in a separating funnel, washed
with water (2 ꢀ 50 mL) followed by saturated sodium bicarbonate solution
(25 mL), and dried over anhydrous sodium sulfate. The solvent was removed
under reduced pressure and the gummy mass was purified by column
chromatography over neutral alumina using gradient elution with pure
hexane to hexane ethyl acetate (9:1) to afford 2-aryl-4-methoxyquinolines
(3a–d) and trans methyl N-acetyl-2-aryl-2,3-dihydroindol-3-carboxylates
(5a–d).
trans Methyl N-acetyl-2-phenyl-2,3-dihydroindol-3-carboxylate (5a):
Colorless thick oil, yield 205 mg (70%). IR (Nujol): v¯ ¼ 1650 (amide),
1
1750cm21 (ester); H NMR (200MHz, CDCl3): d 2.05 (s, 3H), 3.73 (s, 3H),
3.89 (d, 1H, J ¼ 1.7 Hz, H-3), 5.76 (d, 1H, J ¼ 1.7 Hz, H-2), 7.00–7.11
(m, 6H), 7.31–7.35 (m, 2H), 8.31 (brd, 1H, J ¼ 7.14Hz) ppm: MS
(70 eV): m/z ¼ 295 (M+), HRMS: m/z ¼ 295.1208 (calcd. for C18H17NO3
295.1205).
4-Methoxy-2-phenylquinoline (3a): White crystalline solid, yield 26 mg
(11%); mp 66–67 8C (lit.[9] 66–67 8C).
trans
Methyl
N-acetyl-2-(40-methylphenyl)-2,3-dihydroindol-3-
carboxylate (5b): White crystalline solid, yield 215 cm21 (75%);. mp 98–
99 8C; IR (KBr): v¯ ¼ 1640 (amide), 1750 cm21 (ester); H NMR (400 MHz,
1
CDCl3): d 2.05 (s, 3H), 2.29 (s, 3H), 3.75 (s, 3H), 3.93 (d, 1H, J ¼ 1.4 Hz,
H-3), 5.78 (d, 1H, J ¼ 1.4 Hz, H-2), 7.03–7.11 (m, 5H), 7.30–7.34
(m, 2H), 8.31 (brd, 1H, J ¼ 7.2 Hz); 13C NMR (100 MHz, CDCl3): d 21.01
(q), 24.12 (q), 52.80 (q), 55.80 (d, C3), 65.46 (d, C2), 117.18 (d, C7), 124.10
(d, C5), 124.97 (d, C2 & C6,), 125.78 (d, C6) 126.11 (s, C9), 129.30 (d, C4),
0
0
0
0
129.89 (d, C3 & C5 ), 137.97 (s, C4 ), 138.52 (s, C1 ), 143.03 (s, C8),
169.49 (s, C55O, amide), 171.25 (s, C55O, ester); MS (70 eV): m/z ¼ 309
(M+); anal. calcd. for C19H19NO3: C, 73.77; H, 6.19; N, 4.53; found C,
77.85; H, 6.26; N, 4.49.
4-Methoxy-2-(40-methylphenyl)quinoline (3b): White crystalline solid,
yield 25 mg (10%); mp 94–95 8C (lit.[4a] 94–958C).

Ring Contraction of 2-Aryl-l,2,3,4-Tetrahydro-4-Quinolones
949
trans Methyl N-acetyl-2-(40-chlorophenyl)-2,3-dihydroindol-3-carboxy-
late (5c): White solid, yield 230 mg (70%);. mp 52–53 8C; IR (KBr):
1
v¯ ¼ 1660 (amide), 1750 cm21 (ester); H NMR (200 MHz, CDCl3): d 2.01
(s, 3H), 3.71 (s, 3H), 3.86 (d, 1H, J ¼ 1.8 Hz, H-3), 5.77 (d, 1H, J ¼ 1.8 Hz,
H-2), 7.00–7.08 (m, 3H), 7.24–7.32 (m, 4H), 8.24 (brd, 1H, J ¼ 7.2 Hz);
13C NMR (50 MHz, CDCl3): d 24.12 (q), 52.94 (q), 55.68 (d, C3), 64.98
0
(d, C2), 117.23 (d, C7), 124.31 (d, C5), 125.94 (d0, C6), 126.55 (d,0C2 & C6
0
0
and s, C9) 129.49 (d, C3 & C5 ), 134.07 (s, C4 ), 139.96 (s, C1 ), 142.78
(s, C8), 169.20 (s, C55O, amide), 170.96 (s, C55O, ester); MS (70 eV): m/
z ¼ 329/331 (M+); anal. calcd. for C18H16ClNO3: C, 65.56; H, 4.89; N,
4.25; found: C, 65.68; H, 4.82; N, 4.19.
4-Methoxy-2-(40-chlorophenyl)quinoline (3c): White crystalline solid, yield
33 mg (12%); mp 111–112 8C (lit.[4a] 111–112 8C).
trans Methyl N-acetyl-2-(40-methoxyphenyl)-2,3-dihydroindol-3-carboxy-
late (5d): Colorless thick oil, yield 226 mg (70%); IR (Nujol): v¯ ¼ 1650
1
(amide), 1745 cm21 (ester); H NMR (200 MHz, CDCl3): d 2.04 (s, 3H),
3.71 (s, 3H), 3.80 (s, 3H), 3.89 (d, 1H, J ¼ 1.8 Hz, H-3), 5.79 (d, 1H,
J ¼ 1.8 Hz, H-2), 6.92 (d, 2H, J ¼ 8.1 Hz), 7.01 ¼ 7.10 (m, 3H), 7.32–7.37
(m, 2H), 8.29 (brd, 1H, J ¼ 7.3 Hz); MS (70 eV): m/z ¼ 325 (M+); HRMS:
m/z ¼ 325.1312 (calcd. for C19H19NO4 325.1314).
4-Methoxy-2-(40-methoxyphenyl)quinoline (3d): White crystalline solid,
1
yield 32 mg (12%); mp 71–72 8C; IR (KBr): v¯ ¼ 1615 cm21 (C55N); H
NMR (200 MHz, CDCl3): d 3.89 (s, 3H), 4.12 (s, 3H,), 7.04 (brd, 2H,
J ¼ 8.42 Hz), 7.14 (s, 1H), 7.46 (ddd, 1H, J ¼ 8.14, 7.06 & 1.24 Hz), 7.69
(ddd, 1H, J ¼ 8.24, 6.93, & 1.68 Hz), 8.04–8.10 (m, 3H), 8.17 (dd, 1H,
J ¼ 8.19 & 1.74 Hz); MS (70 eV): m/z ¼ 265 (M+); anal. calcd. for
C17H15NO2: C, 76.96; H, 5.70; N, 5.28; found C, 76.91; H, 5.73; N, 5.20.
ACKNOWLEDGMENTS
Authors are grateful to the International Foundation of Science, Sweden and
STEC, Govt. of Kerala (India) for financial support.
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